Acenes, as a class of graphite substructures, are particularly attractive targets in the synthesis of organic semiconductors because of their demonstrated high mobilities, strong intermolecular coupling and small reorganization energies. The usefulness of acene oligomers such as pentacenes are already showing in numerous electronics applications including, but not limited to, thin-film transistors (display technologies), photovoltaic cells and light-emitting diodes.
Substituted acenes have received limited attention due to their synthetic inaccessibility. More specifically, while the properties and limitations of simple, linear conjugated organic systems have been well studied by either synthesis or structure-property determinations performed on series of oligomers, few such studies have been performed on fused aromatic systems, simply because of a lack of synthetic methodology available for their preparation. Although a number of researchers have made excellent approaches to planarized graphitic oligomers and polymers, and simple fused aromatic systems based on the graphite lattice are already being explored for the construction of field effect transistors (FETs) and molecular electronic devices, the lack of a reliable route to synthetically-tailored linearly fused aromatics has precluded the development of fully tunable organic materials.
The ability to tailor organic materials to maximize film-forming abilities or solid-state order cannot be understated, as such customization will allow the use of such systems as components for RFID tags, flexible displays, light-weight solar panels and ubiquitous semiconductor electronics. Functionalization is critical to enable exploration of self-organization in these graphite-like systems. Pendant groups on an oligoacene can be used to alter the solubility, stability and solid-state ordering of the material. Numerous studies of organic semiconductors, including band structure and exchange integral calculations, have shown that subtle changes in semiconductor crystal packing in systems such as the silylethyne-substituted acenes can yield dramatic increases in mobility. See J. E. Anthony et al., J. Phys. Chem. B, 2002, 106, 8288; and J. E. Anthony, et al, Chem. Mater. 2005, 17, 5024.
A number of attempts at modification of packing in current high-performance semiconductors have indeed shown such improvements; for example, alkylation of pentacene, or halogenation of anthradithiophene chromophores led to changes in crystallization or crystal packing that improved performance relative to the parent hydrocarbon. Unfortunately, these approaches require significant additional synthesis steps, and reduce the low-cost advantage promised by organic semiconductors.